1. Field of the Invention
This invention relates to AM stereo receivers designed for reception of a carrier wave having stereo related intelligence appearing in the respective upper sideband and lower sidebands of the transmitted signal, such as disclosed in my prior U.S. Pat. No. 3,218,393, and in my aforesaid copending U.S. Pat. No. 3,908,090. AM stereo receivers according to the invention may incorporate carrier exaltation to reduce signal distortion at low levels of received signal strength and/or inverse amplitude modulation of the carrier and/or quadrature demodulation of the received carrier to derive stereo difference (L -R) signal intelligence and employ inphase detection (e.g. envelope detection or product demodulation) to derive the stero summation (L.+-.R) signal intelligence followed by relative phase shifting and combining of the stereo sum and and difference signals in a manner known per se to produce stereo related (L and R) outputs. Modulation of the carrier wave by an infrasonic frequency (e.g. 15 Hz) is preferably utilized to indicate in the receiver the presence of a stereo modulated signal. Aspects of the invention also relate to specialized receiver circuitry with both stereophonic mode and monophonic mode capabilities and automatic switching therebetween.
2. Description of the Prior Art
Compatible stereophonic AM transmission and reception, involving stereo related upper and lower sidebands, with the difference stereo signal (L-R) intelligence phase modulating the carrier wave and with the summation stereo signal (L+R) intelligence envelope modulating the carrier wave, are disclosed in my U.S. Pat. No. 3,218,393, together with certain forms of receivers for stereophonic reception of a carrier wave so modulated. A further discussion of this compatible AM stereophonic modulation technique appears in my paper entitled "A Stereophonic System For Amplitude Modulated Broadcast Stations", which appears in IEEE Transactions on Broadcasting, Vol. BX-17, No. 2, June 1971, at pages 50-55. To the extent here relevant, the disclosures of this prior patent and this paper are incorporated herein by this reference.
Also known is the so-called "compatible" stereophonic AM system disclosed in Barton U.S. Pat. No. 3,102,167 which in effect utilizes a compromised quadrature modulation technique. To reduce monophonic distortion the Barton system utilizes a relative phase angle between the carrier and sidebands of .+-. 25.degree. to 30.degree., with the two channel signals being developed by use of a phase displaced exalted carrier and product demodulation, but without any combining of the demodulated signals.
Also known are stereophonic transmission and reception systems as disclosed in Shoaf U.S. Pat. No. 3,009,151, involving a two-channel FM-AM stereo system wherein stereo related signals are respectively frequency modulated and amplitude modulated on FM band and AM band carrier waves; Colodny U.S. Pat. No. 3,031,529, disclosing a single channel AM stereo system employing synchronous detectors in the receiver portion of the system; Avins U.S. Pat. No. 3,068,475, disclosing a stereo transmission and reception system wherein one stereo related signal is amplitude modulated on a carrier wave and the other stereo related signal is frequency modulated on the same carrier wave; Fink U.S. Pat. No. 3,206,550, disclosing visual display of a stereo presence signal; Hold et al U.S. Pat. No. 3,167,614, disclosing use of an infrasonic tone to indicate stereo signal presence in an AM/PM type transmission system; and Collins U.S. Pat. No. 3,231,672, disclosing an AM stereo system involving linearly added carrier waves at the same frequency but in different phase, with each of the carrier waves amplitude modulated with stereo related signals.